1. Field of the Invention
The present invention relates generally to a signal wave control circuit, and is directed more particularly to a signal wave control circuit suitable for use in a reproducing system for a digital audio disc record and so on.
2. Description of the Prior Art
At present, a digital-audio disc record has been developed in which in PCM (pulse code modulation) audio signal is recorded on a disc record, which is similar to a video disc, and then reproduced. As the reproducing system thereof, there are known signal detecting systems of a mechanical type, optical type, electrostatic capacity type or the like similar to those for the video disc record. In particular, in the optical of signal detecting system, upon recording a signal on a disc record, a laser which is light-modulated by a recording signal is employed to provide a mastering which is used to produce an original or master disc record formed with pits (or recesses) corresponding to the "1" or "0" bits of the recording signal. Then, a number of optical disc records are reproduced from the master disc record through a pressing process similar to that for an ordinary analog disc record. Depending on the condition of the mastering, there may occur a phenomenon (known as asymmetry) in which the size of the pits is shifted uniformly by a certain amount and hence even though the ON and OFF ratio of the recorded signal is 50%, the ON and OFF ratio of a reproduced signal does not become 50%. Thus, when the reproduced signal is converted to a pulse signal by a wave converting circuit, the pulse width of the pulse signal becomes different from that of the recorded signal, and as a result processes such as the demodulation of reproduced data (for example, the demodulation in the 3 PM system) and so on can not be carried out correctly.
In the art, the above problem is overcome by manually adjusting by an appropriate amount a reference level (or limit level) of a limiter to which the signal read out from the disc record is supplied and which serves as a signal wave converting circuit. Therefore, the prior art circuit requires a complicated adjusting operation.
Further, when a signal is recorded with the base band thereof without employing a carrier modulation system such as amplitude modulation, frequency modulation or the like, a run length limited code modulation system is used. Such a modulation system is used for the purpose that a minimum inversion interval T min between transitions of data between the "0" and "1" levels is selected to be long to increase the recording efficiency, while a maximum inversion interval T max is selected to be short, in order to improve the self clock function in the reproducing system. As an example, the 3 PM system is known in which T min is 1.5 T (where T is the length of a bit cell of input data) and T max is 6 T. Another purpose of shaping the reproduced signal is that when a digital signal is modulated, it is more desirable that the DC component of a modulated digital signal is zero.
FIG. 1 shows a prior art signal wave converting (or control) circuit. With this prior art circuit, a reproduced signal S.sub.p is supplied through an input terminal 1 to one input terminal of a limiter 2 whose output signal S.sub.0 is delivered to an output terminal 3 and also fed to a DC level detecting circuit 4. This DC level detecting circuit 4 may be formed of a low pass filter, an integrating circuit or the like and the output therefrom is fed back to the other input terminal of the limiter 2 as a reference level V.sub.r. This reference level V.sub.r has such a value to set at zero the DC component of the output signal S.sub.0 from the limiter 2.
In operation, a recording signal S.sub.r, which has an ON and OFF ratio of 50% as shown in FIG. 2A, is recorded. If there is no asymmetry, the reproduced signal S.sub.p becomes as shown in FIG. 2B. This reproduced signal S.sub.p does not have an ideal pulse wave shape, due to the influence of the frequency characteristics of transmission paths, the diameter of the read-out beam and so on. The reference level V.sub.r from the DC level detecting circuit 4 is such as will set at zero the DC level of the output signal S.sub.0 from the limiter 2, which is shown in FIG. 2C. Thus, the output signal S.sub.0 can be caused to have an ON and OFF ratio of 50%. However, even if the ON and OFF ratio of the recorded signal is 50%, if there occurs asymmetry, the recording becomes equivalent to one in which the signal is recorded with its pulse width being, for example, expanded as shown in FIG. 3A by the solid line. Thus, the reproduced signal S.sub.p applied to the input terminal 1 becomes as shown in FIG. 3B. Even in this case, since the reference level V.sub.r applied to the limiter 2 has the value to set at zero the DC component of the output signal S.sub.0 from the limiter 2, the output signal S.sub.0 becomes as shown in FIG. 3C which has an ON and OFF ratio of 50%.
As described above, according to the prior art signal wave converting circuit shown in FIG. 1, the problem caused by asymmetry can be automatically overcome. This example assumes however, that for the prior art signal wave converting circuit of FIG. 1 the level of the DC component of the recorded signal does not fluctuates in response to the data, which is required for correct operation. Even if the reference level is to be manually adjusted, the above condition is necessarily premised. With the prior art means, therefore, it is desired that the DC component of the modulated digital signal be held at zero.
To date there has been known almost no satisfactory modulation method in which the minimum inversion interval T min is long and the DC component (or low frequency component) is zero. For any such modulation method, the modulation circuit and demodulation circuit therefor must be complicated in construction.